Well, obviously I don't know what is typical in your area (I'm in the southeast), but I never allowed my son outside with just a pull up on, or for that matter, a pull up and T-shirt only. Not even when he was 3.
If your son doesn't have any sensory reasons that make wearing clothing problematic for him, then why WOULDN'T you want him wearing clothes? I don't get it. City or country makes no difference to me. We were raised on 3 acres of land in the rural part of the county when I was growing up, and I'm quite sure my older brothers weren't outside without clothes on. Ever. (Well, maybe getting mud sprayed off of them right outside the back door before coming inside the house so they didn't get it all over the floors on the way to the bath tub, after playing in the mud.)
Of course he should wear pants. Or shorts. Or *something* other than just a pull-up and T-shirt.
My nephew has Down's Syndrome, and is autistic, and he NEVER is out in public with just a pull-up on. And never has been. And always has had a shirt on, too. He's 9 now (though he is the size of a 5 year old), but there has never been a reason to not expect him to wear appropriate clothing.
My son (who is 18 now) never even ran around the house (except right after baths) without a shirt on. Inside the house.
So, yes. He needs pants on. I'd expect him to have pants on inside, too, not just outside. Is he potty training? So what. My toddler daughter and son both wore clothing over their pull ups when they were potty training. And inside the house. And they potty trained at age 2.
We don't live in a third world country. He should wear pants.